However, have you ever wondered what ensures the stability and clarity of optical signals when they are transmitted across thousands of kilometers of optical fiber? In addition to light sources and amplifiers, there is another type of obscure but crucial device at work-optical passive devices.
They do not require external power, but they undertake key tasks such as controlling the optical path, isolating reflections, and stabilizing signals. This article will combine the content of the picture to deeply explore the core role of passive components in long-distance fiber-optic communication, and reveal how they ensure efficient and reliable signal transmission.
1. What are optical passive components?
Optical passive components (Passive Optical Components) refer to optical devices that do not require external power drive in optical communication systems. They control and adjust optical signals through their own physical structure.
Common passive devices include:
- Isolator
- Circulator
- Collimator
- Polarizer
- Faraday Rotator
Although these devices are simple in structure, they play an irreplaceable role in the system.
2. Challenges of long-distance communication: signal attenuation and reflection interference
In long-distance fiber-optic communication, optical signals face two major challenges:
- Signal attenuation
When optical signals are transmitted in optical fibers, they will gradually attenuate due to absorption, scattering, and other reasons. Although it can be compensated by erbium-doped fiber amplifiers (EDFA), additional insertion loss will be introduced every time a device is passed.
- Reflection interference
Reflected light will be generated at optical fiber connection points, device interfaces, etc. If these reflected lights return to the laser, they will cause laser frequency jitter, increase noise, and even damage the light source.
Therefore, how to reduce loss and suppress reflection has become the key to the design of long-distance communication systems.
3. The core role of passive components
- Isolator: Prevent reflected light from damaging the laser
Fiber isolator is a device that allows light to be transmitted in one direction only. It is usually installed at the output end of the laser to prevent reflected light from returning to the laser cavity, thereby:
Protecting the laser from reflection damage;
Reducing system noise;
Improving signal stability.
For example, the high-power polarization-maintaining isolator mentioned in the picture has excellent performance of **low insertion loss (<0.3 dB) and high return loss (>60 dB), which is very suitable for long-distance, high-power communication systems.
- Circulator: Realizing bidirectional optical path control
Fiber circulator is a three-port device that allows optical signals to be transmitted in a specific direction. It is commonly used in:
Bidirectional communication system: realizing single-fiber bidirectional transmission;
Fiber sensing system: separating incident light from reflected light;
Optical amplifier system: isolating input and output signals to avoid interference.
- Collimator: Improving coupling efficiency and reducing insertion loss
Fiber collimator is used to convert the divergent light beam output by the optical fiber into parallel light, or to couple parallel light into the optical fiber. It plays a key role in the following aspects:
Improving the optical path coupling efficiency;
Reducing the system insertion loss;
Improving the signal transmission quality.
In long-distance systems, every 0.1 dB loss may affect the overall performance, so the accuracy of the collimator is crucial.
- Polarizer and Faraday rotator: Control polarization state and improve system stability
In long-distance transmission, the polarization state of light may change due to factors such as fiber bending and temperature changes. Polarizers and Faraday rotators can:
Stabilize the polarization state;
Reduce polarization-dependent loss (PDL);
Improve the system's anti-interference ability.
4.Typical application cases: The role of passive components in the system
- Trans-sea optical cable system
In trans-oceanic communications, the length of the optical cable can reach thousands of kilometers. Isolators, circulators, and collimators are widely used in the system to ensure that the signal remains high quality after multiple amplifications and transmissions.
- Fiber optic sensor network
In distributed fiber optic sensor systems, circulators are used to separate sensor signals from reference signals, and collimators are used to improve coupling efficiency to ensure that the system can still achieve high-precision measurements over long distances.
- High-power laser communication
In military or industrial high-power communication systems, polarization-maintaining isolators and high-power collimators are key components to ensure stable operation of the system at high power.
Conclusion: Silent protection, stable transmission
In the grand system of long-distance optical fiber communication, although passive components are inconspicuous, they are the "unsung heroes" that ensure stable signal transmission. They silently guard the journey of each beam of light.